Typical aircraft electrical power generation and distribution systems produce and distribute electric power having a constant frequency. These typical systems utilize either a constant speed drive to convert the variable speed of the aircraft engine to a constant speed to drive a generator, or allow the generator to be driven by the engine and convert the variable Frequency output to a constant frequency by an electronic converter. In an effort to reduce overall system cost and weight, several airframe manufacturers are considering the use of variable frequency electric power generation systems to supply all of the electric power required by their aircraft, thereby eliminating the need for a constant speed drive or fully rated electronic converter.
Such variable frequency systems typically comprise variable speed generators driven directly by each of the aircraft's engines. The output power produced by such generators have a frequency which is dependant on the speed of the engine. During the normal cruise flight phase, the output frequency of the generator will be within a fairly narrow band because the engines are at a fairly constant speed. During takeoff, landing, and ground operation, however, the output frequency of the generators will typically vary over a wide band because of the variation in engine speed during these flight phases. While many of the electrical loads will not be adversely affected by such wide variation in frequency, still other more sensitive electrical loads will not tolerate such a wide frequency variation.
To solve the problem of wide frequency variation intolerance by sensitive loads, typical variable frequency generation and distribution systems have included an electronic converter sized and connected to accommodate only the sensitive loads. This converter takes the variable frequency output of the generator, converts this output to a constant frequency, and supplies the constant frequency power to the sensitive loads on their own dedicated load bus. This converter is smaller than a converter rated to supply all of the aircraft loads, and therefore provides advantages over such a system. However, since this dedicated converter is operated during all flight phases, its switching losses reduce the efficiency, and therefore drive up the cost, of the system.
It is an objective of the instant invention to provide a variable frequency electric power generation and distribution system which solves these and other problems in the art. Specifically, it is an objective to provide a variable frequency electric power system which allows the variable frequency generator to provide electrical output to all of the connected loads while its output is within a defined power quality. It is a further objective to provide a system which ensures that sensitive loads continue to receive electric power during all flight phases, and that they do not receive electric power outside the defined power quality during any flight phase. Further, it is an objective of the instant invention to increase efficiency and reduce operating costs of a variable frequency electric power generation and distribution system.